euclid The Time Traveler's Clock, User Manual, Page: 17 / 34

Figure 10: Hamilton 21 Marine Chronometer
GPS Clock Advantages and Deficiencies
The various global navigational satellite systems (GNSS systems, including the US GPS,
Russia’s Glonass, Europe’s Galileo, and China’s Beidou) rely on very-high precision time
keeping in order to allow a receiver to calculate its own position. Each satellite
broadcasts its current time and its ephemeris (the parameters of its orbit). The receiver
must then compare the times received from multiple satellites, correct for relativistic
timing frame differences, and perform a complex calculation to “multilaterate” its own
position on earth. In the process, the receiver also calculates the current time to a very
high degree of accuracy, on the order of nano- or micro-seconds. A timing-rated
receiver can provide true time accurate to within a few tens of nanoseconds.
When provided with an external frequency reference, the GNSS receiver can maintain
the time (“holdover”) even if satellite signals are lost. By itself, the uBlox M8F receiver
has a holdover precision of 1 x 10-
7 , or 8 milliseconds per day. If an atomic reference
signal is supplied, then the holdover precision will be equal to the precision of the
atomic reference signal (in the TTC, 1 x 10 -10 , equivalent to a few milliseconds per year).
Since these satellite signals are available almost anywhere on earth, the time provided
by a GNSS receiver is the time used for most scientific, civil, and navigational purposes.
Why would a user need to use precise time for navigation, given that he has a GNSS
receiver giving precise latitude, longitude, and altitude data? The answer is that GNSS
systems can be jammed or spoofed, resulting in inaccurate location calculations. By
keeping a reliable clock continually set to known good GNSS time, a navigator may use a
sextant and navigational number tables to calculate his position, even if the GNSS
signals are incorrect or unavailable.
The TTC can be used for navigation because it has multiple time references, including
atomic, quartz, and silicon-based clocks in addition to its mechanical chronometer.